The role of two minor seryl-tRNAs from mammalian cells, each of which suppresses the nonsense codon UGA in protein synthesis and forms phosphoseryl-tRNA, is being investigated. Seryl-tRNA synthetase has been purified extensively and the kinase which phosphorylates these seryl-tRNAs is being studied. In addition, it has been found that phenylalanyl-tRNA which lacks the Wye base and occurs in neuroblastoma tissue is used more extensively in protein synthesis than the corresponding phenylalanyl-tRNA which has the fully modified Wye base and occurs in normal tissue.